| Abstract: | The influence of crystallinity on water sorption behavior by poly(vinyl alcohol) (PVA) was studied by using a PVA of low crystallinity (15% crystalline by x-ray analysis) and an annealed sample there from (46%) crystalline. With the increase of crystallinity, the sigmoid shape (which is a characteristic for the sorption isotherm of the low crystalline polymer) diminishes. The B.E.T. plots of the isotherms are linear between the relative vapor pressures of 0.1 and 0.4 as usual, and deviate from straight lines in the higher pressure range in the direction of a larger sorbed quantity than that predicted by the B.E.T. theory. This tendency is regarded as a kind of dissolution, and the Flory-Huggins interaction parameter χ1 was calculated. In both polymers, the χ1 versus pressure relation has a maximum, while overall χ1 values are smaller in the polymer of low crystallinity. The maximum point (which lies in the higher pressure region in case of the less crystalline sample) is considered to be a transition point from a phenomenon controlled mainly by an adsorption mechanism to a phenomenon controlled mainly by a dissolution mechanism. Accordingly, the separation of the isotherm into adsorption and a dissolution components was made, and the polymer fraction which contributes to the dissolution mechanism versus pressure relation was calculated. The result indicates that the crystalline region observed by x-ray analysis may partly contribute to the dissolution process at room temperature. |
